Tuesday, March 3, 2009

About a month ago, we noticed that Quark, a 5-month old Happy Zebra cub, had a very swollen right eye. The swelling has finally gone down, and now we realize the eye isn’t just damaged…it’s completely gone.

I know what you’re thinking: are you guys really geeky enough to name a lineage of hyenas after subatomic particles?

Um, yes we are. But perhaps you’re also thinking: yikes…what’s it like being a hyena with only one eye?

Hyenas have what we call “binocular vision,” meaning that their vision is a result of two overlapping images – one from each eye – that the brain puts together. This allows for several fantastic advantages, such as depth perception and increased detection. Animals such as predators (which rely on particular visual cues to hunt moving prey) and primates (which need to find and select particular fruits from the surrounding vegetation) often have binocular vision. Their eyes are usually placed closely together on the head and oriented forward.

Animals with “monocular vision” (like the rhino below) have eyes that are on opposite sides of their head, rather than in front. These animals use each eye separately, but the big advantage here is an increased field of view – sometimes nearly 360 degrees. If you’re a tasty Mara antelope and the biggest problem in your life is some scary carnivore creeping up on you while you’re chowing down on grass, you’re going to want to see as much as the world around you as possible. For most prey species, accuracy and depth perception probably aren’t quite as important as knowing what's around you at all times.

OK, enough biology lessons. The point is that, since hyenas need abilities such as depth perception, poor one-eyed Quark is probably at a real disadvantage. Try closing one eye and trying to play darts (first, please make sure nobody else is within striking distance)…it’s not easy. Now, imagine losing an eye and trying to catch a gazelle zipping around at 50 miles per hour.

But, Quark still has one good eye, and we aren’t counting her out quite yet. After all, our resident rebel Moss is likely blind in one eye, and she continues to kick some serious butt. Plus, Quark is pretty high-ranking, so, in theory, she doesn’t need to hunt at all to survive…she just needs to take advantage of others’ success. In any case, life probably isn't going to be easy for Quark.

Now I'm wondering though, why does the Rhino have it's eyes to the sides?

Maybe a grazer just doesn't need binocular vision.. so the ancestral state is retained in the Rhino (which should be a smallish herbivore then).. Binocular vision necessitates special brain wiring on top of the more visible adaptations, so I guess binocular vision should be considered a specialization, more so than eyes to the sides.

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